In conventional color electrophotography a series of electrostatic images are created on an image member. They are toned with different color toners and then transferred in registration to a receiving surface to create a multicolor toner image. Typically, the receiving surface is a receiving sheet of paper or similar material which has been secured around the periphery of a transfer roller. The transfer roller is rotated in contact or near contact with the image member to repeatedly bring the receiving sheet into transfer relation with the consecutive images to overlay them in registration.
As color toners get finer, higher resolutions become possible. A limiting aspect of the process is the image registration provided by the transfer process.
Some transfer processes bring the receiving sheet into light contact or just out of contact with the image member, and transfer is accomplished by an electrostatic field. Using encoders and separate motors on both the image member and the transfer roller, accurate registration from image to image is possible with such systems. See, for example, U.S. Pat. Nos. 4,796,054 and 4,872,037.
However, fine toners do not generally transfer well electrostatically. Transfer systems using substantial amounts of pressure, sometimes in the presence of sufficient heat to soften or sinter the toner have been more successful in transferring fine toners. Further, for highest quality work, a receiving sheet with a heat-softenable thermoplastic outer layer can be used to receive the toner in the presence of sufficient heat to soften the outer layer and soften or sinter the toner.
These transfer processes require more pressure than is common with ordinary electrostatic transfer. Unfortunately, if both the transfer roller and the image member are independently driven at more than light pressures, excessive wear will destroy a normal image member, for example, a photoconductive member, quite rapidly.
If the transfer roller is driven by the image member, such wear does not occur but registration is difficult to maintain. Even with precisely machined devices, there is a drift from image to image and also over a number of images. Even slight misregistrations which would not be objectionable in an ordinary color copier, are objectionable in a higher quality print. Also of significance, as the registration drifts, the image may be transferred out of registration with the means for holding the receiving sheet to the periphery of the transfer roller, for example, vacuum holes or gripping fingers.
The above-referenced U.S. application Ser. No. 07/488,546 suggests that more precise registration can be obtained if the transfer roller is separated from the transfer drum between transfer of each image and reindexed. It further suggests using a stepper motor for that reindexing and a cam which rides on the image member or a roller backing it for separating the transfer roller and image member.
The apparatus described in this prior patent application does, in fact, provide improved registration without excessive wear to the image member.